Polyarylates or polyesters derived from bisphenol-A and iso/terephthalic acids are tough materials having a continuous use temperature of about 140.degree. C. However, as in the case with many other amorphous thermoplastics, they possess poor solvent and stress-crack resistance. Such polyesters may be represented as having repeating units as shown below: ##STR1##
To improve the solvent and stress-crack resistance, a variety of copolyarylates utilizing other diphenolic and/or acid reactants, were prepared. Thus, significantly improved materials were obtained by substituting part of the bisphenol-A with 4,4'-biphenol, as described in U.S. Pat. No. 4,275,188. Another interesting class of polyarylates are those that contain hydroquinone. Hydroquinone-containing polyarylates are described in, for example, U.S. Pat. Nos. 3,036,990; 3,036,991; 3,036,992; 3,160,605; 3,251,804; and in Japanese Patent Application 74/21,498. Bisphenol-A/hydroquinone iso/terephthalates were first described in Vysokomol. Soedin. 1 (10), 1482 (1959); the same materials can also be found in U.S. Pat. Nos. 3,216,970 and 4,334,053. Compositions containing 5 to 35 mole percent of hydroquinone were transparent and displayed good stress crack and chemical resistance, and good thermal stability. These compositions were described in U.S. Pat. No. 4,064,108. Bisphenol-A/hydroquinone copolyesters were also prepared in U.S. Pat. No. 4,436,894; however, only the isophthalate of bisphenol-A and hydroquinone was described in this latter patent.
Japanese patent application 57/164,120 describes copolyarylates comprising units (2) and (3) in molar ratios of (2)/(2)+(3)=0.2 to 0.8, ##STR2## where Ar' is a bisphenol residue of the formula ##STR3## Y being a divalent residue such as --CH.sub.2 --, --C(CH.sub.3).sub.2 --, --C(CF.sub.3).sub.2 --, --O--, --S--, --SO.sub.2 --, or --CO--; m and n being 0 or 1; with the proviso that n cannot be 0 when m is 0; or of the formula: ##STR4## Y being as previously defined; and R.sub.1, R.sub.2, R.sub.3, and R.sub.4 being C.sub.1 to C.sub.4 alkyl, alkoxy or halogen groups, and where Ar" is ##STR5## R being a C.sub.1 to C.sub.4 alkyl, alkoxy or halogen group, and r being an integer of 0 to 4. According to the application, the moiety ##STR6## comprises preferably 100 to 90 mole percent of para- and 0 to 10 mole percent of meta-carbonyl groups.